This invention relates to a method and apparatus for detecting an invisible object, such as an underground buried object, by transmitting an electromagnetic wave and receiving an echo wave from the object.
FIG. 1 is a schematic view showing a measuring system for detecting an underground buried object using an electromagnetic wave. An electric pulse which is sent from pulse generator 1 is fed to transmitting antenna 2 where it is radiated as an electromagnetic wave into the underground. The radiated electromagnetic wave is reflected on an object, such as water layer 17, stone 18, underground buried piping 5, 13, and returned as an echo wave to receiving antenna 6. In a practical measuring apparatus, transmitting and receiving antennas 14 are formed as one system. The echo wave is fed via sampling unit 7 to computing unit 9 where it is computed. Then the result of computation is sent to display unit 10.
FIG. 2 shows an example of a measurement observation signal which is incident to receiving antenna 6 moved over underground buried piping 5. In FIG. 2, the abscissa represents a time unit which needs to be calculated in terms of the distance. It is, therefore, necessary to compute the velocity of a wave which is propagated across the underground. The velocity is given by: ##EQU1## wherein:
c: velocity of the electromagnetic wave propagated through a vacuum space; and
.epsilon..sub.s : dielectric constant of the earth.
In this connection it is to be noted that it is necessary to measure the dielectric constant .epsilon..sub.s of the earth at the measuring site. The dielectric constant .epsilon..sub.s of the earth is generally 4 to 20. The abscissa in the graph of FIG. 2 denotes the round trip time of an electromagnetic wave between the ground surface and the object. In this case, the depth L is expressed as follows: ##EQU2## where T represents the time of an electromagnetic wave propagated between the ground surface and the object.
In actual practice, antenna system 14 including transmitting antenna 2 and receiving antenna 6 is scanned in a direction of arrow 105 of FIG. 1 and an observation signal is stored in computing unit 9 each time the antenna is moved a distance of 2 cm. At the completion of scanning, the distance is calculated in terms of the depth according to Equation (2) and observation signals are sequentially arranged in the order in which they are picked up as shown in FIG. 3 (the abscissa: scanning distance; the ordinate: depth of the earth). The amplitude of the signal is stepwise distinguished, usually in color, in accordance with the level of the amplitude to form an underground cross-sectional image. FIG. 3 is an example of an underground cross-sectional image as displayed on display unit 10 of FIG. 1. In the graph of FIG. 3, the abscissa denotes the distance as set forth above and the ordinate denotes the depth of the earth, noting that the dielectric constant of the earth at a detection site was found to be 16 upon measurement and so calculated and that the observation signal of FIG. 2 corresponds to a distance A--A in FIG. 3. In the underground cross-sectional image shown in FIG. 3, a time position for those observation signals of an amplitude of over 30 mV and a time position under 30 mV are indicated by a cross-hatched area and white area, respectively. That is, the amplitude 30 mV is the minimum discrimination amplitude I min of the observation signal.
In order to distinguish the echo waves returned back from the object to be detected, such as buried piping or water layer, from a superimposed signal of various echo waves, the amplitude magnitudes of the observation signals are employed as a discrimination standard. As shown, for example, in "Applied Science Center for Archaeolog-MASCA Newsletter vol 11 NO2 December 1975, echo wave 11 whose amplitude is greater than a minimum discrimination width Imin as shown in FIG. 2 has been employed as an echo wave returned back from the object, noting that wave 12 denotes a ground surface echo wave. In the case where, for example, there are some objects, such as stones 18, which are distinct from the object to be detected, that is, some causes for strongly returning the magnetic wave, a plurality of echoes 19, 20 appear in the underground cross-section as shown in FIG. 3. This makes it very difficult to distinguish the echo wave coming from the object to be detected, from the plurality of echoes of objects.
It is accordingly an object of the present invention to provide an object detection method and apparatus which can positively and accurately detect an object of interest even if there are a plurality of the other object of objects from which an echo wave or waves are returned back.